THE STUDY SUBJECTS INCLUDED 86 TALL (6 FEET OR TALLER) INMATES AT ATTICA (N.Y.) STATE PRISON, 30 TALL MALE WARDS AT GOWANDA STATE HOSPITAL, 30 TALL CONTROLS, 15 MALE JUVENILES CONVICTED OF OFFENSES THAT MERITED DETENTION PENDING COURT ACTION, AND 42 MALE JUVENILES REFERRED BY A COURT FOR PSYCHIATRIC EXAMINATION. THE JUVENILES WERE NOT SELECTED FOR HEIGHT. THE XYY KARYOTYPE, ABNORMAL CHROMOSOME CONFIGURATION ASSOCIATED WITH TALLNESS AND POSSIBLY WITH BEHAVIORAL PROBLEMS WAS FOUND IN TWO INMATES, ONE PSYCHIATRIC PATIENT, ONE DELINQUENT, AND NO CONTROLS. OTHER THAN UNUSUAL HEIGHT, THE XYY SUBJECTS SHOWED NO SIGNIFICANT PHYSICAL ABNORMALITIES. THE HIGH PREVALENCE OF BROKEN HOMES AND OTHER ADVERSE ENVIRONMENTAL FACTORS IN THE XYY SUBJECTS' CHILDHOODS WAS STRIKING. THERE WAS ALSO A HIGH PREVALENCE OF MENTAL SUBNORMALITY AMONG THE SIBLINGS OF THE XYY SUBJECTS. THE POSSIBILITY THAT THE DEVELOPMENT OF ANTISOCIAL, AGGRESSIVE, OR CRIMINAL BEHAVIOR IN XYY INDIVIDUALS MIGHT BE AVERTED THROUGH THE PROVISION OF SOCIAL AND PSYCHIATRIC SUPPORT TO THEIR FAMILIES IS CONSIDERED. CASE STUDIES OF THE XYY SUBJECTS, SUPPORTING DATA, AND A LIST OF REFERENCES ARE PROVIDED. (LKM)